1. Field of the Invention
The present invention relates to a positive resist composition for use in lithographic steps in the production of semiconductors, e.g., IC's, in the production of circuit boards for liquid crystals, thermal heads, etc., and in other photofabrication processes, and further relates to a method of pattern formation with the same, In particular, the invention relates to a positive resist composition suitable for exposure with an immersion exposure type projection exposure apparatus employing far ultraviolet rays having a wavelength of 300 nm or shorter as an exposure light, and to a method of pattern formation with the composition.
2. Description of the Related Art
With the trend toward size reduction in semiconductor elements, the wavelengths of exposure lights are decreasing and the numerical apertures (NA) of projection lenses are increasing. An exposure apparatus which has an NA of 0.84 and employs an ArF excimer laser having a wavelength of 193 nm as a light source has been developed so far. As is generally well known, resolution and focal depth can be expressed by the following equations:(Resolution)=k1·(λ/NA)(Focal depth)=±k2·λ/NA2
wherein λ is the wavelength of the exposure light, NA is the numerical aperture of the projection lens; and k1 and k2 are coefficients relating to the process.
An exposure apparatus employing an F2 excimer laser having a wavelength of 157 nm as a light source is being investigated for the purpose of enhancing resolution by using a shorter wavelength. However, use of this apparatus is disadvantageous in that materials for the lens to be used in the exposure apparatus and materials for resists are considerably limited due to the use of such a shorter wavelength. Because of this, the cost of apparatus and material production is high and it is exceedingly difficult to stabilize quality. There is hence a possibility that an exposure apparatus and a resist which have sufficient performances and stability might be not available in a desired period.
The so-called immersion method has been known as a technique for enhancing resolution in examinations with optical microscopes. In this method, the space between the projection lens and the sample is filled with a liquid having a high refractive index (hereinafter referred to also as “immersion liquid”).
This “immersion” has the following effects. In the immersion, the resolution and the focal depth can be expressed by the following equations on the assumption that NA0=sin θ:(Resolution)=k1·(λ0/n)/NA0(Focal depth)=±k2·(λ0/n)/NA02wherein λ0 is the wavelength of the exposure light in air; n is the refractive index of the immersion liquid relative to that of air; and θ is the convergence half angle of the light.
Namely, the immersion produces the same effect as the use of an exposure light having a wavelength reduced to 1/n. In other words, in the case of an optical projection system having the same NA, the focal depth can be increased to n times by the immersion. This is effective in all pattern shapes and can be used in combination with a super resolution technique such as the phase shift method or the deformation illumination method.
Examples of apparatus in which this effect is applied to the transfer of fine circuit patterns for semiconductor elements are described in JP-A-57-153433, JP-A-7-220990, etc. However, no resist suitable for the immersion exposure technique is discussed therein.
JP-A-10-303114 points out that a change in refractive index of the immersion liquid causes deterioration of projected images due to the wave surface aberration for the exposure apparatus and, hence, it is important to regulate the refractive index of the immersion liquid. To regulate the temperature coefficient of refractive index of an immersion liquid to a value within a certain range is disclosed in this document. Water containing an additive which serves to lower surface tension or heighten surface activity is also disclosed therein as a preferred immersion liquid. However, in this document also, additives are not specifically disclosed and no resist suitable for the immersion exposure technique is discussed.
JP-B-63-49893, a pattern forming method based on the liquid immersion process is disclosed.
WO 2004/068242A1 specification, there is disclosed a resist composition, which is used for a resist pattern formation method including a liquid immersion exposure step, characterized by that the increment in film thickness when immersed in water does not exceed 1.0 nm.
WO 2004/074937A1 specification, materials are disclosed for forming a resist-protecting film suited for the liquid immersion exposure process to be provided on a resist film.
Recent progress in the immersion exposure technique is reported in SPIE Proc, 4688, 11(2002), J. Vac. Sci. Technol., B 17(1999), etc. In the case where an ArF excimer laser is used as a light source, pure water (refractive index at 193 nm, 1.44) is thought to be most promising from the standpoints of safety in handling and transmittance and refractive index at 193 nm. Although solutions containing fluorine are being investigated for use in the case of using an F2 excimer laser as a light source from the standpoint of a balance between transmittance and refractive index at 157 nm, no immersion liquid has been found which is sufficient from the standpoints of environmental safety and refractive index. In view of the degree of the effect of the immersion and the degree of completion of resists, the technique of immersion exposure is thought to be employed first in ArF exposure apparatus.
Since the advent of resists for KrF excimer lasers (248 nm), the technique of image formation called chemical amplification has been used as a resist image formation method for compensating for a sensitivity decrease caused by light absorption. For example, the chemical amplification type method for forming a positive image comprises exposing a resist film to light to thereby cause an acid generator in the exposed areas to decompose and generate an acid, subjecting the resist film to post-exposure bake (PEB) to utilize the resultant acid as a reaction catalyst to convert alkali-insoluble groups into alkali-soluble groups, and removing the exposed areas by alkali development.
When a chemical amplification type resist is applied to the technique of immersion exposure, the acid which has generated upon exposure and is present in the resist surface moves to the immersion liquid to change the acid concentration in the surface of the exposed areas. This is thought to be considerably akin to the acid deactivation in exposed-area surfaces which is caused by basic contaminants which have come from the environment in an exceedingly slight amount on the order of ppb during a time delay between the exposure and PEB (PED: post-exposure time delay); the acid deactivation was a serious problem in the initial stage of the development of chemical amplification type positive resists. However, influences of immersion exposure on the resist and the mechanism thereof have not been elucidated.
In immersion exposure, the space between the resist film and the optical lens is filled with an immersion liquid (hereinafter sometimes referred to as an “immersion liquid”) and the resist film in this state is exposed to light through a photomask to thereby transfer the pattern of the photomask to the resist film. It is expected that the immersion liquid infiltrates into inner parts of the resist film to thereby influence the chemical reactions to be induced in the resist during or after the exposure (acid-catalyzed protection-eliminating reaction and development reaction). However, the degree and mechanism of this influence also have not been elucidated.
For example, when a chemical amplification type resist showing satisfactory PED stability in ordinary exposure is used in immersion exposure, then resist pattern falling and profile deterioration occur as a result of a time delay between exposure and PEB. An improvement in this point has been required.